Formable lightweight steel having improved mechanical properties and method for producing semi-finished products from said steel

ABSTRACT

The invention relates to a formable lightweight steel having improved mechanical properties and a high resistance to delayed hydrogen-induced cracking formation and hydrogen embrittlement comprising the following elements (in wt. %): C 0.02 to ≤1.0; Mn 3 to 30; Si≤4; P max. 0.1; S max. 0.1; N max. 0.03; Sb 0.003 to 0.8, particularly advantageously to 0.5, as well as at least one or more of the following carbide-forming elements in the specified proportions (in wt. %): Al≤15; Cr&gt;0.1 to 8; Mo 0.05 to 2; Ti 0.01 to 2; V 0.005 to 1; Nb 0.005 to 1; W 0.005 to 1; Zr 0.001 to 0.3; with the remainder consisting of iron including the usual steel-accompanying elements, with the optional addition of the following elements, in wt. %: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu. 
     The invention also relates to a method for producing the said lightweight steel.

The invention relates to a formable lightweight steel having improved mechanical properties and a high resistance to delayed, hydrogen-induced crack formation, according to claim 1. The invention further relates to a method for producing semi-finished products from this steel.

“Semi-finished product” is understood hereinafter to mean a hot or cold strip produced from this steel or an intermediate or final product produced therefrom, such as tubes for example.

In recent years, there have been many advances in the field of so-called lightweight steels which are characterised by a low specific weight whilst maintaining high strength and toughness, and have a high ductility and are thus of great interest for vehicle construction.

In these steels which are austenitic in the initial state, a reduction in weight, which is advantageous for the automotive industry, is achieved whilst maintaining the previous mode of construction owing to the high content of alloy components (Si, Al) having a specific weight far below the specific weight of iron.

The deformable lightweight steel known from the laid open document DE 10 2004 061 284 A1 has e.g. the following alloy composition (in wt. %): C 0.04 to ≤1.0, Al 0.05 to <4.0, Si 0.05 to ≤6.0, Mn 9.0 to <18.0, with the remainder being iron including typical steel-associated elements. Optionally, Cr, Cu, Ti, Zr, V and Nb can be added as required.

This lightweight steel has a partly stabilised γ mixed crystal microstructure having defined stacking fault energy with a partly multiple TRIP effect which permits the stress-induced or expansion-induced conversion of a face-centred γ mixed crystal (austenite) into an ε martensite (hexagonal densest sphere packing) and then upon further deformation into a body-centred ε martensite and residual austenite.

The high degree of deformation is achieved by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) properties of the steel.

However, in this and comparable steels delayed embrittlement triggered by hydrogen and, as a result thereof, crack formation can occur in the presence of residual stresses in the material depending upon the microstructure and strength.

To overcome this problem, laid open document DE 10 2004 061 284 A1 already proposed to limit the hydrogen content to <20 ppm, preferably to <5 ppm.

Although this proposal is helpful, it is not yet sufficient because the effect of the delayed crack formation can still occur even when the hydrogen contents are set low. Moreover, in steel production it is possible that the fixed maximum value of hydrogen is exceeded for various reasons, which can be tolerated in terms of the alloy but increases the risk of hydrogen embrittlement.

An austenitic steel is known from laid open document WO 2011/154153 A1 and is said to have an excellent resistance to delayed crack formation. In addition to iron and impurities, the steel contains, in wt. %: 0.5 to 0.8 C, 10 to 17 Mn, at least 1.0 Al, at most 0.5 Si, at most 0.020 S, at most 0.050 P, 50 to 200 ppm N and 0.050 to 0.25 V.

A steel alloy for a high-strength, cold-rolled steel sheet is known from laid open document WO 2009/142362 A1 and is likewise said to have an improved resistance to delayed crack formation. In addition to iron and impurities, the steel contains, in wt. %: 0.05 to 0.3 C, 0.3 to 1.6 Si, 4.0 to 7.0 Mn, 0.5 to 2.0 Al, 0.01 to 0.1 Cr, 0.02 to 0.1 Ni, 0.005 to 0.03 Ti, 5 to 30 ppm B, 0.01 to 0.03 Sb and 0.008 or less S.

Furthermore, a lightweight steel having an improved expansion is known from laid open document EP 2 128 293 A1 and comprises, in addition to iron and impurities, in wt. %: 0.2 to 0.8 C, 2 to 10 Mn, 0.2 or less P, at most 0.015 S, 3.0 to 15 Al, at most 0.01 N and a ratio Mn/Al of 0.4 to 1.0.

Furthermore, a method for continuously heat-treating a steel strip is described in laid open document US 2009/0050622 A1, the strip thickness thereof varying along its length. This steel strip with varying thickness is produced continuously by so-called flexible rolling. In this regard, a nip of a roller system is varied in a targeted manner during the production of the steel strip.

The object of the invention is to provide a lightweight steel of the generic type which does not have the effect of a delayed crack formation or hydrogen embrittlement whilst maintaining very good mechanical properties (ductility, strength).

This object is achieved, proceeding from the preamble, in conjunction with the characterising features of claim 1 and, in relation to a method, by the features of claim 6. Advantageous developments are described in the dependent claims.

According to the teaching of the invention, the formable lightweight steel having TRIP and TWIP properties comprises the following elements in wt. %:

C 0.02 to ≤1.0 Mn 3 to 30 Si≤4 P max. 0.1 S max. 0.1 N max. 0.03

Sb 0.003 to 0.8, advantageously to max. 0.5 and at least one or more of the following carbide-forming elements in the stated proportions (in wt. %):

Al≤15 Cr>0.1 to 8 Mo 0.05 to 2 Ti 0.01 to 2 V 0.005 to 1 Nb 0.005 to 1 W 0.005 to 1 Zr 0.001 to 0.3

with the remainder being iron including typical steel-associated elements, with the optional addition of the following elements in wt. %: max. 5 Ni, max. 10 Co, max. 0.005 Ca, max. 0.01 B and 0.05 to 2 Cu.

Surprisingly, it has been found during extensive testing that by alloying antimony (Sb) at the stated limits the diffusion of elements, in particular C, N and O is impeded and as a result the material behaviour can be advantageously influenced in conjunction with a targeted heat treatment.

The addition of antimony results in the carbides growing more slowly and thus being distributed more finely and being precipitated to a smaller size. As a result, alloy elements are used more effectively which results in more cost-favourable alloy concepts with improved mechanical properties and a clear improvement in terms of avoiding delayed hydrogen-induced crack formation (delayed fracture) and hydrogen embrittlement.

It has proved to be favourable if the ratio of Sb/C does not exceed a value of 1.5. Values above 1.5 do not provide any further advantage in terms of the invention and primarily produce negative effects such as e.g. a loss of ductility and toughness owing to precipitation of antimony at the grain boundaries.

In accordance with the invention, the mechanical properties are evaluated by determining the tensile strength and elongation at fracture of the product, which is a measurement for the performance of the material.

It has been shown in tests that in the case of the alloys in accordance with the invention, the tensile strength and elongation at fracture are considerably higher owing to the addition of antimony compared with steel alloys to which no antimony is added, whereby steels can be produced which are more cost-favourable and are of higher value.

It has also been demonstrated that the above-described effect of antimony can be considerably increased by heat-treating the steel.

In order to obtain a further improvement in the required properties, the product or semi-finished product, which is produced from the alloy in accordance with the invention by deformation and may be e.g. a hot strip, cold strip, flexibly rolled hot or cold strip, a tube or a vehicle body component, is thus advantageously subjected to a heat treatment at 480 to 770° C. for 1 minute to 48 hours, e.g. in a batch-type annealing process with predominantly long annealing times or in a continuous annealing process with predominantly short annealing times.

However, such annealing can also already take place prior to the final shaping to form a finished product, e.g. on the cold strip which will be subsequently further processed. The timing of the annealing can thus be adapted in a flexible manner to the production process. Annealing the final product in addition to earlier annealing of the semi-finished product can result in a further improvement of the material properties.

Furthermore, the invention is accomplished by a method for producing the steel in accordance with the invention with the following steps:

-   -   casting the steel in a continuous casting process or thin-slab         casting process or a horizontal or vertical strip casting         process approximating the final dimensions,     -   hot rolling the cast slab or the cast strip with a thickness of         more than 5 mm to a unitary thickness or flexibly hot rolling         the cast slab or the cast strip with a thickness of more than 5         mm to different thicknesses,     -   optionally cold rolling the hot strip rolled to a unitary         thickness or the cast strip—which is produced by means of a         casting process approximating the final dimensions and is at         most 5 mm thick—to a unitary thickness or optionally flexibly         cold rolling the hot strip rolled to a unitary thickness or the         cast strip—which is produced by means of a casting process         approximating the final dimensions and is at most 5 mm thick—to         different thicknesses,     -   optionally annealing the hot strip or cold strip with the         following parameters:     -    annealing temperature: 480 to 770° C., annealing duration: 1         minute to 48 hours.

In relation to the cast strip which is produced by means of a casting process approximating the final dimensions and is at most 5 mm thick, it is particularly advantageous if this is cold-rolled to a unitary thickness or is flexibly cold-rolled to different thicknesses and then if the cold strip is annealed with the following parameters: annealing temperature: 480 to 770° C., annealing duration: 1 minute to 48 hours.

In alloys with Al contents of >1 wt. %, the annealing treatment is preferably carried out at temperatures of 670 to 770° C. at annealing times of 1 minute to 12 hours, because lower temperatures and longer annealing times result is a lower tensile strength and elongation at fracture.

For the annealing itself, for a hot strip, cold strip and flexibly rolled strips a continuous annealing process is preferably used for short annealing times and a batch-type annealing process is preferably used for long annealing times. For other products and semi-finished products, other annealing devices having the predetermined parameters, such as e.g. a muffle furnace, can be used.

By means of the invention, the production of cost-favourable Sb-alloyed steels having a higher content of manganese is possible, said steels having improved tensile strength and elongation at facture compared with non-Sb-alloyed steels having a higher content of manganese with the same chemical composition.

Moreover, by adding antimony the behaviour with respect to hydrogen (delayed crack formation and hydrogen embrittlement) is also considerably improved.

The improvement in the material properties is caused by the antimony impeding the diffusion of carbon and aluminium. Furthermore, antimony decreases the interfacial energy which results in the carbides being distributed more finely. The reduced carbon diffusion thus delays the local enrichment of carbon at the grain boundaries and in the microstructure and in conjunction with aluminium the forming of kappa-carbides or in particular with V, Nb, Mo, Cr, W, Zr and Ti the forming of local larger carbides. The homogeneity of the material is thus improved with the described positive effects on the mechanical properties and the resistance to delayed crack formation and hydrogen embrittlement. The precipitation of finely distributed carbides results in grain refinement in the microstructure which is associated with an improvement in the behaviour with respect to hydrogen-induced negative effects (delayed crack formation, hydrogen embrittlement) and an increase in the strength and improvement in the toughness and expansion properties.

Owing to the addition of small amounts, up to max. 0.8 wt. %, of antimony in accordance with the invention, the behaviour of the material with respect to hydrogen-induced influences is thus considerably improved.

In contrast, the addition of excessive amounts of antimony causes an undesirably strong precipitation of antimony at the grain boundaries and thus reduces the toughness and expansion properties. In order for antimony to be able to be effective, proportions of at least 30 ppm are required. However, antimony contents of over 0.8 wt. % embrittle the material and are thus to be avoided. Optimally, the maximum content of antimony is 0.5 wt. %.

The small carbides which are precipitated in a much more finely distributed manner compared with the prior art (predominantly Cr-, Mo-, Ti-, Nb-, V-, W-, Zr- and kappa-carbides) improve the efficiency of the corresponding alloy elements which potentially allows a reduction in the amount added. Furthermore, the reduced carbon diffusion and the reduced grain growth owing to the alloying of antimony increase the process window for the heat treatments required in accordance with the invention, i.e. the steel reacts in a less sensitive manner to process fluctuations (temperature, time) in relation to the resulting mechanical properties.

The positive effects of the alloy elements used in accordance with the invention will be described hereinafter:

Al: improves the strength and expansion properties, decreases the specific density and influences the conversion behaviour of the alloys in accordance with the invention. Contents of Al of more than 15 wt. % impair the expansion properties for which reason a maximum content of 15 wt. % is set. High Al contents of greater than or equal to 4 wt. % act in conjunction with high C contents of greater than or equal to 0.6 wt. % as carbide forming agents for kappa-carbides. At less than 4 wt. %, Al delays the precipitation of carbides.

B: Improves the strength and stabilises the austenite. Contents of more than 0.01 wt. % result in embrittlement of the material for which reason a maximum content of 0.01 wt. % is set.

C: is required to form carbides, stabilises the austenite and increases the strength. Contents of more than 1 wt. % C impair the welding properties and result in the precipitation of undesirably large carbides and thus in the impairment of expansion and toughness properties for which reason a maximum content of 1 wt. % is set. In order to achieve a sufficient strength for the material, a minimum addition of 0.01 wt. % is required.

Ca: used to modify non-metallic oxidic inclusions which can lead to inhomogeneities and an undesired material failure. Owing to its high vapour pressure in liquid steel, the content is limited to at most 0.005 wt. %.

Co: increases the strength of the steel, stabilises the austenite and improves the heat resistance. Contents of more than 10 wt. % impair the expansion properties for which reason a maximum content of 10 wt. % is set.

Cr: improves the strength and reduces the rate of corrosion, delays the formation of ferrite and perdite and forms carbides. The maximum content is set to 8 wt. % since higher contents result in an impairment of the expansion properties.

Cu: reduces the rate of corrosion and increases the strength. Contents of above 2 wt. % impair the producibility by forming low melting point phases during casting and hot rolling for which reason a maximum content of 2 wt. % is set.

Mn: stabilises the austenite, increases the strength and the toughness and permits a deformation-induced martensite formation and/or twinning in the alloys in accordance with the invention. Contents of less than 3 wt. % are not sufficient to stabilise the austenite and thus impair the expansion properties whilst no further advantages are to be expected for contents of greater than 30 wt. % and the production is made more difficult owing to the low Mn vapour pressure.

Mo: acts as a strong carbide forming agent and increases the strength. Contents of Mo of more than 2 wt. % impair the expansion properties for which reason a maximum content of 2 wt. % is set.

Nb+V: act in a grain-refining manner in particular by forming carbides, whereby at the same time the strength, toughness and expansion properties are improved. Contents of more than 1 wt. % do not provide any further advantages.

Ni: stabilises the austenite and improves expansion properties in particular at low application temperatures. An addition of more than 5 wt. % of Ni does not provide any further advantages.

Si: Impedes the diffusion of carbon, reduces the specific density and increases the strength and expansion properties and toughness properties. Furthermore, an improvement in the cold-rollability can be seen by alloying Si. Contents of more than 4 wt. % result in embrittlement of the material and negatively influence the hot- and cold-rollability for which reason a maximum content of 4 wt. % is set.

Ti: acts in a grain-refining manner as a carbide forming agent, whereby at the same time the strength, toughness and expansion properties are improved and the inter-crystalline corrosion is reduced. Contents of Ti of more than 2 wt. % impair the expansion properties for which reason a maximum content of 2 wt. % is set.

W: acts as a carbide forming agent and increases the strength and heat resistance. Contents of W of more than 1 wt. % impair the expansion properties for which reason a maximum content of 1 wt. % is set.

Zr: acts as a carbide forming agent and improves the strength. Contents of Zr of more than 0.3 wt. % impair the expansion properties for which reason a maximum content of 0.3 wt. % is set.

Advantageous alloy combinations are shown in claims 3 to 5.

An alloy as claimed in claim 3 has, using optimised heat treatment parameters (see tables 1 to 4), a product of tensile strength and elongation at fracture of at least 20,000 MPa % and a tensile strength of at least 800 MPa. The product of tensile strength and elongation at fracture is a measurement for the performance of the material upon deformation.

Although the heat treatment at 680° C. for 10 min in table 2 still does not provide optimum values for the product of tensile strength and elongation at fracture of at least 20,000 MPa %, the positive effect of alloying antimony can already be seen here.

An alloy as claimed in claim 4 has a product of tensile strength and elongation at facture of at least 30,000 MPa % and a tensile strength of at least 800 MPa.

An alloy as claimed in claim 5 has finely distributed kappa-carbide precipitations and a product of tensile strength and elongation at facture of at least 30,000 MPa % and a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa.

The examined alloy compositions are provided in table 1. The content of Sb and additions of Nb are varied, with the remaining chemical composition being approximately identical.

Hot strips with a thickness of 2 mm were produced from these steels and then cooled in air after hot rolling. Test pieces were removed from these hot strips and the tensile strength and elongation at fracture were determined thereon.

The results of the product of tensile strength and elongation at fracture are shown in tables 2 to 4, wherein the heat treatment having the highest product of tensile strength and elongation at fracture is considered to be the most favourable for the respective alloy. It is clear that the steels alloyed with Sb in accordance with the invention always have a higher product of tensile strength and elongation at facture than the comparative alloys.

TABLE 1 Alloy composition Alloy C Mn Al Si Cr V Sb other L1 0.19 7.1 2 0.55 1 0.05 0 L2 0.19 7.1 2 0.55 1 0.05 0.012 L3 0.19 7.1 2 0.55 1 0.05 0.027 L4 0.19 7.1 2 0.55 1 0.05 0.041 L5 0.21 6.3 2 0.2 1 0.06 0 Nb 0.05 L6 0.21 6.3 2 0.2 1 0.05 0.039 Nb 0.05 L7 0.25 7.9 1 0.5 0.9 0.08 0 L8 0.25 7.9 1 0.5 0.9 0.08 0.04

TABLE 2 determined products of tensile strength and elongation at fracture L1 to L4 Heat TS*El treatment L1 L2 L3 L4 650° C., 24 h 22453 23195 23772 22633 680° C., 10 min 15263 16695 16830 16111 680° C., 5 h 27162 27997 28258 29000 680° C., 24 h 26660 28985 30546 29720

TABLE 3 determined products of tensile strength and elongation at fracture L5 and L6 Heat TS*El treatment L5 L6 690° C., 3 h 19368 21449 750° C., 10 min 22751 25502 500° C., 10 min 23525 26737

TABLE 4 determined product of tensile strength and elongation at fracture L7 and L8 Heat TS*El treatment L7 L8 650° C., 24 h 18378 20457 

1.-8. (canceled)
 9. A formable lightweight steel having improved mechanical property and high resistance to delayed hydrogen-induced crack formation and hydrogen embrittlement, the formable lightweight steel comprising the following elements, in wt.-%: C 0.02 to ≤1.0 Mn 3 to 30 Si≤4 P max. 0.1 S max. 0.1 N max. 0.03 Sb 0.003 to 0.8, and at least one carbide-forming element selected from the group consisting of, in wt.-% Al≤15 Cr>0.1 to 8 Mo 0.05 to 2 Ti 0.01 to 2 V 0.005 to 1 Nb 0.005 to 1 W 0.005 to 1 Zr 0.001 to 0.3, with the remainder being iron, including typical steel-associated elements.
 10. The formable lightweight steel of claim 9, wherein a proportion of Sb is 0.003 to 0.5.
 11. The formable lightweight steel of claim 9, further comprising at least one element, in wt-%: Ni max. 5; Co max. 10; Ca max. 0.005; B max. 0.01; and Cu 0.05 to
 2. 12. The formable lightweight steel of claim 9, wherein a ratio Sb/C is less than or equal to 1.5.
 13. The formable lightweight steel of claim 9, wherein the elements have the following proportions, in wt-%: C is 03 to 0.5 Mn 3 to 10 Al 0.1 to 4 Si 0.1 to 3 Sb 0.005 to 0.3 Cr>0.1 to 5 V 0.005 to 1, wherein the steel has a product of tensile strength and elongation at facture of at least 20,000 MPa % and a tensile strength of at least 800 MPa.
 14. The formable lightweight steel of claim 9, wherein the elements have the following proportions, in wt-%: C 0.1 to 0.35 Mn 5 to 9 Al 1 to 3.5 Si 0.1 to 1 Sb 0.01 to 0.1 Cr 0.5 to 4 V 0.02 to 0.1, wherein the steel has a product of tensile strength and elongation at facture of at least 20,000 MPa % and a tensile strength of at least 800 MPa.
 15. The formable lightweight steel of claim 9, wherein the elements have the following proportions, in wt.-%: C 0.4 to 0.9 Mn 12 to 18 Al 0.5 to 4 Si 0.5 to 3 Sb 0.005 to 0.4, said at least one carbide-forming element being selected in the following proportions (in wt.-%): Cr>0.1 to 4 Mo 0.05 to 1 Ti 0.01 to 0.1 V 0.005 to 0.3 Nb 0.005 to 0.3 W 0.005 to 0.5 Zr 0.001 to 0.3, with the remainder being iron, including typical steel-associated elements, wherein the steel has a product of tensile strength and elongation at facture of at least 30,000 MPa % and a tensile strength of at least 800 MPa.
 16. The formable lightweight steel of claim 9, wherein the elements have the following proportions, in wt.-%: C 0.6 to 1.4 Mn 10 to 30 Al>4 to 15 Si 0.05 to 0.5 Sb 0.005 to 0.5, said at least one carbide-forming element being selected in the following proportions (in wt.-%): Cr>0.1 to 4 Mo 0.05 to 1 Ti 0.01 to 0.1 V 0.005 to 0.3 Nb 0.005 to 0.3 W 0.005 to 0.5 Zr 0.001 to 0.3, with the remainder being iron, including typical steel-associated elements, wherein the steel has finely distributed kappa-carbide precipitations and a product of tensile strength and elongation at facture of at least 30,000 MPa % and a yield strength of at least 700 MPa and a tensile strength of at least 800 MPa.
 17. A method for producing a formable lightweight steel having improved mechanical properties and a high resistance to delayed hydrogen-induced crack formation and hydrogen embrittlement, the formable lightweight steel comprising the following elements, in wt.-%: C 0.02 to ≤1.0 Mn 3 to 30 Si≤4 P max. 0.1 S max. 0.1 N max. 0.03 Sb 0.003 to 0.8; and at least one carbide-forming element selected from the group consisting of, in wt.-%: Al≤15; Cr>0.1 to 8; Mo 0.05 to 2; Ti 0.01 to 2; V 0.005 to 1 Nb 0.005 to 1 W 0.005 to 1 Zr 0.001 to 0.3, with the remainder being iron, including typical steel-associated elements, the method comprising: casting the lightweight steel in a continuous casting process, a thin-slab casting process, to form a cast strip or a cast slab with a thickness of more than 5 mm, or a horizontal or vertical strip casting process approximating the final dimensions to form a cast strip with a thickness of at most 5 mm; hot rolling the cast slab or cast strip with a thickness of more than 5 mm to a uniform thickness, or flexibly hot rolling the cast slab or cast strip to different thicknesses.
 18. The method of claim 17, further comprising: after hot-rolling, cold rolling the hot-rolled strip having the uniform thickness or cold rolling the cast strip, which has a thickness of at most 5 mm and is produced by a casting process approximating the final dimensions, to a uniform thickness or flexibly cold-rolling to different thicknesses.
 19. The method of claim 17, further comprising: after hot-rolling, annealing the hot-rolled strip or cold-rolled strip at an annealing temperature of 480 to 770° C. and an annealing duration of 1 minute to 48 hours.
 20. The method of claim 18, further comprising: after hot-rolling, annealing the hot-rolled strip or cold-rolled strip at an annealing temperature of 480 to 770° C. and an annealing duration of 1 minute to 48 hours.
 21. The method of claim 17, further comprising: after hot-rolling, cold-rolling the cast strip, which has a thickness of at most 5 mm and is produced by a casting process approximating the final dimensions, to a uniform thickness or flexibly cold-rolling the cast strip to different thicknesses and then annealing the cold strip with the following parameters: annealing temperature: 480 to 770° C., annealing duration: 1 minute to 48 hours.
 22. The method of claim 17, wherein, when the steel has a proportion, in wt-%, of Al>1, the annealing temperature is 670 to 770° C. and the annealing duration is 1 minute to 12 hours. 